Water softeners of the "ion exchange"-type typically include a resin tank through which hard water passes to exchange its "hard" ions of calcium and magnesium for "soft" sodium ions from the resin bed. Regeneration of the resin bed is required periodically to remove the accumulation of hard ions and replenish the supply of soft ions. Regeneration is typically effected by flushing a brine solution through the resin bed. A water softener of this type is more fully described in U.S. Pat. No. 3,891,552 issued Jun. 24, 1975 to William Prior and James W. Kewley, entitled CONTROL VALVE FOR WATER SOFTENERS.
Modern water softeners of the type disclosed in U.S. Pat. No. 3,891,552 typically employ a brine tank which includes a reservoir and supply of salt disposed at a level above the bottom of the reservoir. A tube connected to a source of water establishes a path for water to flow to the reservoir. Upon the attainment of a predetermined level in the reservoir, the water reacts with the salt supply to produce a source of brine for regeneration of the resin bed. When regeneration is required, the brine is aspirated through the same tube that supplied water to the reservoir. The amount of water introduced to the brine tank after the regeneration cycle and the amount of brine aspirated from the tank during a regeneration cycle is controlled by a brine valve mechanism.
Commercially available water softeners generally include one or two tanks which contain the softening chemicals that form the resin beds. In one type of two-tank water softener, one tank is regenerated and kept "off-line" while the other tank is "on-line." A control valve controls the communication of the tanks with the household water supply and controls the timing and sequence of regeneration. An example of such a control valve is disclosed in U.S. Pat. No. 3,891,552. An improved control valve is described in U.S. Pat. No. 4,298,025. Both of these patents are owned by the present assignee and are herein incorporated by reference.
A prior art resin tank typically comprises an elongate cylinder in which the ion exchange resin is contained. A conduit, often called a riser pipe, extends downwardly from the top of the tank. A filter screen, mounted at the end of the conduit prevents the entry of resin into the conduit. An opening is formed in the top of the tank for discharging or admitting fluid depending on the direction of the flow.
In many applications, prior art water softeners such as the one described in the above referenced patent, perform satisfactorily. However, in some areas of the country "problem water" is encountered which is not easily treated by conventional water softening apparatus. In particular, in some regions, the water has a high iron content and a relatively low pH. When a standard water softening apparatus is used to treat this type of water, the result is usually considered unacceptable or marginally acceptable.
Methods and apparatus are known or have been suggested for treating water having a high iron content and low pH. Examples of specialized treatment systems are disclosed in U.S. Pat. Nos. 4,764,280 and 4,804,465, both of which are owned by the assignee of the subject application and are hereby incorporated by reference. Industrial type apparatus and methods are also known for treating problem water but, many if not most are considered uneconomical for home use or use a process that the average homeowner is not equipped to handle.
Ion exchange resins are also known which are especially effective for use in treatment systems for "problem" water. One such resin is available from the Rohm & Haas Company and is identified as Amberlite.RTM. DP-1 resin.
Amberlite.RTM. DP-1 cation exchange resin and other carboxylic acid exchange resins have the following general selectivity sequence, H.sup.+ &gt;Ca.sup.++ &gt;Mg.sup.++ &gt;Na.sup.+. This selectivity sequence is reflected in the buffering capacity of the Na.sup.+ form of the cation exchange resin in water with a pH between 5-7 and the readiness of the resin to exchange Na.sup.+ for Ca.sup.+ + or Mg.sup.+ +. The buffering capacity of the cation exchange resin allows the use of the resin in a water softener in applications where the source water has a pH between 5-7 without requiring the use of caustic feeders or calcite neutralizers.
When used in a water softener, Amberlite.RTM. DP-1 will exchange with cations in aqueous solution by the following reaction: ##STR1##
Buffering of acidic water, containing dissolved carbon dioxide (CO.sub.2), occurs by the following reaction: R--COO.sup.- Na.sup.+ +H.sub.2 O+H.sub.2 CO.sub.3 --&gt;R--COOH+NaHCO.sub.3 +H.sub.2 O Sodium bicarbonate, NaHCO.sub.3, as a 0.1 molar solution, has a pH of 8.4.
Regeneration of Amberlite.RTM. DP-1, because of the resin's unusual affinity for divalent and trivalent cations and buffering ability, has previously been regenerated with strong acid/alkali solutions or a solution of sodium citrate or sodium fumarate. Amberlite.RTM. DP-1 cannot be regenerated with brine in the same manner as conventional, sulfonic acid, water ion exchange resins are regenerated.
In Rohm and Haas U.S. Pat. No. 4,071,446, 4,116,860 and 4,083,782 , methods and compositions for regenerating sodium carboxylate ion exchange resins such as Amberlite.RTM. are described and are hereby incorporated by reference. Two of the methods described in these references for regenerating the ion exchange resin back to the sodium carboxylate form are the acid/alkali system and the sodium citrate or sodium fumarate systems.
The acid/alkali regeneration process is generally composed of the following steps:
1. Optional back wash; PA1 2. Acid regeneration, wherein a dilute sulfuric acid solution or preferably a hydrochloric acid solution is used to wash the resin to convert the carboxylate ion back to the acid form and dissolve any accumulated iron or other metal oxides; PA1 3. Water rinse; PA1 4. Caustic feeder wherein a caustic solution such as sodium hydroxide, sodium carbonate or sodium bicarbonate in an amount sufficient to neutralize from 70-96% of the theoretical exchange capacity of the resin is passed through the exchange bed. PA1 5. Slow rinse. PA1 6. Hold. PA1 7. Final Rinse. PA1 8. Final back wash.
The sodium citrate or sodium fumarate system is a one step regenerating composition that is safe for a residential environment. The regenerating solution for a residential water system would be a mixture of 75 weight percent sodium chloride, 15 weight percent sodium citrate and 10 weight percent sodium carbonate. The regenerating solution is created using pellets in a conventional manner in the same way that sodium chloride pellets are use to create brine solution in a more conventional water softening system.